Add option to not collect statistics for long char/varchars

ANALYZE TABLE can take a long time and use a lot of disk space when trying to do statistic for long char/varchar columns.

In most cases, having statistics for long char/varchar is is not usable.
ANALYZE TABLE has options to limit which columns should be used. It is however not practical to provide a set of columns for many tables.

MDEV-21829 will reuse the needed disk space a bit, but will only be available in 12.0 or later.

This task is to add a new server variable that will specify the max length for char/varchar variables that will be included in analyze table.
Any column listed in analyze table will be included in the statistics independent of their lengths.

The name of the new variable is analyze_max_length